1. Field of the Invention
The present invention generally relates to the recovery of bitumen from tar sands. More particularly, the present invention relates to a method and apparatus for efficient solvent extraction of bitumen from viscous tar sands.
Tar sands, also known as bituminous sands and oil sands, are primarily aggregates of sand, clay, oil and, sometimes, water. Tar sands are capable of being surface mined. There are large known reserves of heavy petroleum locked up in tar sand deposit in various parts of the world, including the United States.
One of the best known tar sand deposits is the Athabasca tar sands in Alberta, Canada. It has been estimated that Athabasca contains recoverable reserves in excess of 250 billion barrels of oil. This quantity is significant because the proven world wide oil reserves is on the order of 850 billion barrels. At the present rate of consumption, there is less than 40 year supply of oil. Therefore, there is a need for a process capable of producing oil from tar sand in sufficiently large quantifies in an economical and environmentally acceptable manner.
The Athabasca tar sands contain 5 to 20 percent heavy oil by weight. This constitutes approximately one barrel of oil per 2 tons of sand. The Athabasca tar sands also contain fine particles of clay and silt which can range from 1 to 50 percent by weight, and between 1 and 10 percent water, depending upon the amount of clay present in the tar sand. The average water content is approximately 6 percent.
As mentioned above, tar sand deposits are also known to exist in the United States. One of the largest U.S. deposits is the Sunnyside deposit in Utah. As compared to Athabasca, Sunnyside is relatively small, containing reserves estimated at 1 to 2 billion barrels of bitumen. Generally, the U.S. deposits of tar sand differ from the Athabasca deposit in that the U.S. tar sands do not have a coating of water along with the coating of bitumen. Consequently, the hot water process used for the Athabasca tar sands cannot be used for U.S. deposits.
2. Description of the Prior Art
There are scores of patents that disclose diverse processes and apparatus for recovering oil from tar sands. For example, U.S. Pat. No. 4,046,668, entitled "Double Solvent Extraction of Organic Constituents from Tar Sands," discloses a method wherein the extraction of hydrocarbons from tar sands is performed with a light naphtha/methanol solvent system. The patent specification claims that the system results in rapid precipitation of a tar-sand aggregate and separation of the organic matter into three phases. Non-polar organic materials are recovered from the light naphtha phase, more polar soluble constituents are recovered from the methanol phase and less desirable asphaltene constituents are separated as a precipitate. The drawback with this process is that it requires the use of two solvents which tends to increase the costs of processing a unit of tar sand.
In another example, U.S. Pat. No. 4,347, 118, entitled "Solvent Extraction Process for Tar Sands," discloses a solvent extraction process wherein a low boiling solvent having a normal boiling point from 20.degree. to 70.degree. C. is used to extract tar sands. Initially, the solvent is mixed with tar sands in a ratio of approximately 0.5:1 to 2:1 in a dissolution zone. The mixture is passed to a separation zone in which bitumen and inorganic fines are separated from extracted sand. The separation zone comprises a classifier and a countercurrent extraction column. The extracted sand is introduced into a first fluid-bed drying zone fluidized by heated solvent vapors, where unbound solvent is extracted from sand and the water content of the sand is lowered to less than approximately 2 weight percent. The so-treated sand is passed into a second fluid-bed drying zone fluidized by a heated inert gas to remove bound solvent. Recovered solvent is recycled to the dissolution zone. The major drawback of this process is that the drying process using fluidized beds is not energy efficient. Also, according to the patent, good fluidization requires lowering the moisture content of the extracted sand to less than 2 weight percent.
Finally, in yet another example, U.S. Pat. No. 5,143,598 entitled "Methods of Tar Sand Bitumen Recovery," discloses a method comprising the steps of mixing tax sand, solvent and a displacing amount of aqueous medium to form a mixture. A bitumen-rich solvent phase is separated from the mixture and the bitumen therein is recovered. The mixture is separated into a bitumen-rich solvent phase. The primary drawback with this process is that the solvent recovery process using steam vaporization and-condensation is energy inefficient.
One of the best known and commercially successful prior art processes is the "hot water method." In the "hot water method," tar sand is treated with steam and thoroughly mulled and aerated in a large rotating drum with hot water. The resulting pulp is dropped into a turbulent stream of circulating hot water and carried to a separation cell wherein the sand settles to the bottom and the oil rises to the top in the form of a frothy mixture of minerals and oil. The frothy mixture is mixed with naphtha and fed to centrifuges to separate water and clay. The "hot water method" has several serious disadvantages. First, the separation of oil from the sand is difficult, and even with a great number of separation stages, complete recovery of the oil cannot be obtained. Second, large quantifies of process water, on the order of 150 to 250 gallons per ton of tar sand processed is required. For example, the production of 200,000 barrels of oil per day requires approximately 60 to 100 million gallons of process water per day. This large quantity of process water represents large heat losses and disposal problems. Furthermore, oil cannot be recovered in sufficient quantities needed to satisfy the demand due to the limited availability of water at the tar sand site. While some of the water from the "hot water processes" may be recycled back into the process after treatment in settling ponds, the quantity recycled is limited by the large areas of land needed for the settling ponds because the process water typically contains suspended fines.
There are various other prior art methods proposed in patents that utilize a hydrocarbon solvent to extract oil from the tar sands. These methods appear attractive because separating the bitumen from the sand is relatively easy. However, a major problem with such processes is the difficulty with recovering the solvent contained in large volumes of sand. Also, these prior art processes require large amounts of solvent to form a slurry with the tar sand feed and the energy requirement for recovering solvent can be high. Furthermore, the solvent capable of dissolving all components of the bitumen (generally oil, resin, and asphaltenes), must also be compatible with the refinery operations.